This invention relates to an electrical contact composition for a vacuum type circuit interrupter used in a high current circuit with voltages above 10 KV.
The desirable characteristics of an electrical contact for a vacuum type circuit interrupter include the following:
(1) a low welding force, PA1 (2) the ability to withstand high voltages, PA1 (3) a large interrupting current capacity, PA1 (4) a low chopping current, and PA1 (5) minimal contact erosion.
In actual practice it is difficult for a contact to meet all of these requirements, and consequently some of the more essential requirements or characteristics are favored at the sacrifice of the others, in the sense of a trade off.
Conventional contacts are typically made by a fusion process or a powder metallurgy process, in both of which various kinds of second phase materials are added to copper (Cu), which is the principal phase material. The amount of second phase material added may be either greater or less than its solid solubility limit in copper, and it may have a higher or lower melting point than copper.
Conventional contacts may be roughly classified into two types depending on whether or not an added material will increase the overall contact brittleness, bismuth (Bi) being a typical material for increasing the brittleness of a copper based contact. Bismuth is only slightly soluble in copper, has a lower melting point than copper, and it itself relatively brittle. In a copper-bismuth (Cu-Bi) contact, the bismuth tends to segregate at the boundaries between the crystals of copper, and consequently such a contact has a low tensile strength. Such a contact has an excellent (low) welding force characteristic, however, and can thus be used in a high current circuit. Tellurium (Te), antimony (Sb) and certain other elements are also effective in increasing brittleness, but they are not as effective as bismuth for this purpose.
While contacts containing materials for increasing brittleness can be used in high current circuits, as mentioned above, they are mainly used in circuits ranging from 3 to 6 KV because of their relatively poor ability to withstand high voltages.
A typical prior art contact which does not contain materials for increasing brittleness is made by dispersing chromium (Cr) into a principal phase material of Cu (See the copending U.S. Patent Appln. Ser. No. 910,905 filed on May 26, 1978 by M. Kato) or a Cu-Cr solid solution. Such a Cu-Cr contact satisfies most of the above requirements, and may be used in circuits with voltages higher than 10 KV. This type of contact exhibits a large welding force, however, and consequently it cannot be used in a high current circuit.
Other high voltage contacts include iron (Fe) or cobalt (Co) dispersed into a principal phase material of Cu or a Cu-Fe solid solution, or into Cu or a Cu-Co solid solution, respectively. These types of contacts also do not contain any material for increasing brittleness, however, so they still have the defect of a large welding force characteristic.
Atomic ratios of these solid solutions as the first phase material are usually as follows. Atomic ratios of Cr: Cu, Fe: Cu, Co: Cu are less than 0.8, 4.5 5.5 (w%) respectively.